Rebuild config "extends" field

src/cli.ts

@@ -1,9 +1,10 @@
 #!/usr/bin/env node
+import color from "ansi-colors";
+import createMatcher from "ignore";
 import { readFile } from "node:fs/promises";
 import { join, resolve } from "node:path";
 import parseArgs from "yargs-parser";
-import createMatcher from "ignore";
-import color from "ansi-colors";
+import { getConfig } from "./config.js";
 import { PackageJson, PackageLockJsonV3 } from "./types.js";
 
 const args = parseArgs(process.argv.slice(2), {
@@ -17,11 +18,13 @@ const context = resolve(args.context ?? ".");
 
 const packageFilename = join(context, "package.json");
 const packageInfo: PackageJson = JSON.parse(await readFile(packageFilename, "utf-8"));
-const config = packageInfo.nawCheckNpmDependencies;
-if (!config) {
+const configJson = packageInfo.nawCheckNpmDependencies;
+if (!configJson) {
 	throw new Error("\"nawCheckNpmDependencies\" field is missing package.json");
 }
 
+const config = await getConfig(context, configJson);
+
 const packageLockFilename = join(context, "package-lock.json");
 const packageLock: PackageLockJsonV3 = JSON.parse(await readFile(packageLockFilename, "utf8"));
 if (packageLock.lockfileVersion !== 3) {
@@ -67,7 +70,7 @@ for (const path in packageLock.packages) {
 	});
 }
 
-if (config.noDuplicates) {
+if (config.noDuplicates.length > 0) {
 	const matcher = createMatcher({ ignoreCase: false });
 	matcher.add(config.noDuplicates);
 	for (const [name, group] of groupsByName) {
@@ -84,7 +87,7 @@ if (config.noDuplicates) {
 	}
 }
 
-if (config.sameVersions) {
+if (config.sameVersions.length > 0) {
 	for (const pattern of config.sameVersions) {
 		const matcher = createMatcher({ ignoreCase: false });
 		matcher.add(Array.isArray(pattern) ? pattern : [pattern]);

src/config.ts

@@ -0,0 +1,45 @@
+import { readFile } from "node:fs/promises";
+import { dirname, resolve } from "node:path";
+
+export interface ConfigJson {
+	extends?: string;
+	noDuplicates?: string[];
+	sameVersions?: (string | string[])[];
+}
+
+export interface Config {
+	noDuplicates: string[];
+	sameVersions: (string | string[])[];
+}
+
+export async function getConfig(context: string, json: ConfigJson | string): Promise<Config> {
+	const config: Config = {
+		noDuplicates: [],
+		sameVersions: [],
+	};
+
+	async function follow(filename: string): Promise<void> {
+		const json = JSON.parse(await readFile(filename, "utf-8"));
+		use(dirname(filename), json);
+	}
+
+	async function use(context: string, json: ConfigJson): Promise<void> {
+		if (json.extends) {
+			await follow(resolve(context, json.extends));
+		}
+		if (json.noDuplicates) {
+			config.noDuplicates?.push(...json.noDuplicates);
+		}
+		if (json.sameVersions) {
+			config.sameVersions?.push(...json.sameVersions);
+		}
+	}
+
+	if (typeof json === "string") {
+		await follow(resolve(context, json));
+	} else {
+		await use(context, json);
+	}
+
+	return config;
+}

src/types.ts

@@ -1,11 +1,7 @@
-
-export interface Config {
-	noDuplicates?: string[];
-	sameVersions?: (string | string[])[];
-}
+import { ConfigJson } from "./config.js";
 
 export interface PackageJson {
-	nawCheckNpmDependencies?: Config;
+	nawCheckNpmDependencies?: ConfigJson | string;
 }
 
 export interface PackageLockJsonV3 {